Fix using in-memory PIL Image objects without a backing file

Created originally on Bitbucket by alexei.popelane

I’ve added a minimal test for the fix, and run through the test suite locally. I’m not sure what “configure pipelines for your account” actually implies; I’ve toggled pipelines to enabled for my fork, but I’m not sure it’s actually reading the existing bitbucket-pipelines.yaml file. Or does that need modifications?